Conventionally, as this sort of acceleration sensors, such an acceleration sensor has been proposed in, for example, JP-A-2003-14778, which corresponds to U.S. Pat. No. 6,739,189-B2. That is, while a semiconductor substrate made by stacking a silicon semiconductor layer on a supporting substrate is prepared, since a trench etching process operation is carried out along a first direction (namely, a vertical direction of substrate plane) perpendicular to a substrate plane of the semiconductor substrate with respect to the silicon semiconductor layer, both a movable electrode and a fixed electrode are patterned and formed on the silicon semiconductor layer along a second direction equal to a parallel direction of the substrate plane, and the movable electrode is located opposite to the fixed electrode with having a detection interval.
Then, when acceleration is applied to this acceleration sensor along the first direction, the movable electrode is displaced along the first direction, and this applied acceleration is detected based upon a capacitance change between the movable electrode and the fixed electrode in conjunction with this displacement.
In other words, the opposite area between the movable electrode and the fixed electrode is changed due to the displacement of the movable electrode along the first direction in conjunction with the application of the acceleration along the first direction. As a result, the capacitance formed between the movable electrode and the fixed electrode is also changed, so that the acceleration can be detected. As previously explained, this sort of acceleration sensor constitutes a vertical displacement type acceleration sensor which detects the acceleration applied along the first direction, namely along the vertical direction of the substrate plane, based upon the displacement of the movable electrode along the same direction.
Furthermore, in this acceleration sensor, lengths of the movable electrode and the fixed electrode opposite to this movable electrode along the vertical direction are different from each other. That is, a thickness of the movable electrode along the first direction is made different from a thickness of the fixed electrode along the first direction.
This condition is caused by the following reason. That is, since the thickness of the movable electrode is different from the thickness of the fixed electrode along the vertical direction of the substrate plane while these electrodes are located opposite to each other along the parallel direction of the substrate plane, changing degrees of the opposite area between the movable electrode and the fixed electrode due to the displacement of the movable electrode when the acceleration is applied along the vertical direction of the substrate plane are different from each other along one direction and the other direction in the vertical direction. As a consequence, the direction of the acceleration applied along the vertical direction of the substrate plane can be properly detected.
If the thickness of the movable electrode is made equal to the thickness of the fixed electrode along the vertical direction of the substrate plane while these electrodes are located opposite to each other along the parallel direction of the substrate plane, then changing degrees of the opposite area between the movable electrode and the fixed electrode due to the displacement of the movable electrode when the acceleration is applied along the vertical direction of the substrate plane are identical to each other along one direction and the other direction in the vertical direction. As a consequence, the direction of the acceleration applied along the vertical direction of the substrate plane cannot be properly detected, so that this sensor structure is not proper as such an acceleration sensor.
On the other hand, as described in JP-A-2003-14778, in this acceleration sensor, the movable electrode and the fixed electrode are patterned by trench-etching the silicon semiconductor layer along the vertical direction of the substrate plane.
Then, the opposite interval between the movable electrode and the fixed electrode is made wider or narrower in this etching process operation. Namely, the width of the removing portion by the etching process operation is made wider or narrower. As a result, the etching rates along the vertical direction of the substrate plane are different from each other in order that the movable electrode and the fixed electrode are patterned, and the thicknesses of the movable electrode and the fixed electrode opposite thereto are made different from each other along the vertical direction of the substrate plane.
However, in the conventional acceleration sensor, when the movable electrode and the fixed electrode are patterned by way of the trench etching process operation, since a difference between the etching rates, namely a so-called “RIE lag” is utilized, reproducibility of the etching process operation is largely fluctuated.
As a result, the resulting thicknesses of the movable electrode and the fixed electrode along the vertical direction of the substrate plane are fluctuated, so that the opposite area between the movable electrode and the fixed electrode is largely fluctuated, and thus, the sensor characteristic is fluctuated, i.e., deviated.